The long-term objective of this Program Project is to advance the understanding of the pathogenic process in prion diseases, with a special focus on elucidating the mechanisms by which these diseases are transmitted. The major specific goals are: (i) Determine structural mechanisms of prion protein conformational conversion and prion propagation; (ii) Advance molecular level understanding of prion strains and transmissibility barriers; (iii) Elucidate molecular and structural determinants that control transmissibility and phenotypic variability of familial and atypical forms of human prion diseases. Answers to questions addressed in this research are of fundamental importance for understanding the pathogenic process and from a more practical perspective of pharmacological intervention and public health safety. The Program has been structured into three integrated and interactive Research Projects supported by three Cores. Each individual Research Project addresses different, yet closely related, aspects of the overall goals, approaching them using different experimental tools and at different levels of organization and complexity. The main focus of Research Project 1 is on elucidating the structural basis of prion protein conversion and the structure of the infectious PrPSc conformer. This Project also seeks to identify specific structura differences in PrPSc that underlie phenotypic variability of human prion diseases. Research Project 2 focuses on issues related to prion strains and transmissibility barriers as well as prion therapy. Using PMCA technology, structural studies and bioassays, this Project seeks to (i) elucidate the mechanisms involved in controlling barriers for prion transmissibility from animals to humans; (ii) explore the mechanism by which the specific environment of different organs affects the selection of prion strains and their replication potential; (iii) develop a novel therapeutic strategy in prion disease. The focus of Research Project 3 is on human prion diseases, seeking to (i) develop better understanding of mechanisms controlling phenotypic variability of these disorders; (ii) understand the role of prion protein glycosylation in some fors of these diseases and (iii) elucidating the biochemical and structural basis of large differences i transmissibility between different forms of human prion diseases (in collaboration with Project 1). Each of these three Research Projects will be supported by the Administrative, Animal and Neuropathology Cores that will provide common services with regard to research administration and coordination, all aspects of animal studies, and preparation and neuropathological characterization of animal and human tissue.